Edit: I'm told he's a Western Oriole.
I thought it was a goldfinch, but there are goldfinches on the seed-feeder which are much smaller. And this has a yellow forehead and black breast. The goldfinches are supposed to have a black forehead and a yellow breast.
Here's a hummingbird for size comparison.
Whoever he is, he's a big meanie. He chased away all the little hummers.